1. Field of the Invention
The present invention relates to the processing of video signals and more particularly to the adjustment of the black level of a video signal with respect to a reference level.
2. Discussing of the Related Art
FIG. 1 very schematically represents a portion of a video signal CVBS. This signal includes, below a reference level B, so-called black level, periodic line synchronization pulses S. Between two consecutive pulses S, there sequentially occurs a constant level equal to B, a portion of the active signal conveying line information, and again a constant level equal to B. Each active portion of the video signal lasts for approximately 52 microseconds, and the inactive portion, corresponding to the remaining period, lasts for approximately 12 microseconds. Each synchronization pulse lasts for approximately 4.7 microseconds.
In devices receiving such video signals, such as television sets, it is desired, to suitably use the video signals, to adjust the black level B with a predetermined reference level. This is required, for example, to adjust with a same reference level the black levels of a plurality of video signals to be switched on a channel, or to extract from the video signals the synchronization pulses S that serve as a reference to a phase-looped loop (PLL). Such a PLL serves, more particularly, to synchronize scanning of the screen with pulses S.
FIG. 2 is a general diagram of a very commonly used, so-called I/8I or I/7I, conventional device that serves to adjust the black level of a video signal with a reference voltage Vref. The video signal CVBSi to be adjusted is applied to a first terminal of a capacitor C. The second terminal, A, of capacitor C is connected to a low potential, such as ground, through a current source I, and selectively to a high potential Vcc through a current source 8I that is controlled by a switch K. Terminal A is also connected to the inverting input of a comparator 10, whose non-inverting input receives a reference voltage Vref. Comparator 10 controls switch K. The adjusted video signal, CVBSo, is drawn from terminal A.
When signal CVBSo exceeds voltage Vref, switch K is off, and capacitor C is discharged by a constant current I. When signal CVBSo is below the reference voltage Vref, switch K is on and capacitor C is charged at a constant current 7I. Terms "charge" and "discharge" are used to indicate that the potential of terminal A is pulled up to the high potential Vcc and pulled down to ground, respectively, which does not systematically correspond to actual charging or discharging of capacitor C.
With this configuration, signal CVBSo tends to approach an equilibrium state where the time duration of its portion higher than Vref, hereinafter referred to as "positive half-period", is 7 times longer than the time duration of its portion lower than Vref, hereinafter referred to as "negative half-period". In other words, the ratio between the positive and negative half-periods of signal CVBSo tends to approach the ratio between the charging and discharging currents.
Referring back to FIG. 1, and assuming that the black level B is slightly lower than value Vref, the signal has 52-microsecond positive half-periods and 12-microsecond negative half-periods. The ratio between these periods is equal to 4.3. Assuming that the black level B is slightly higher than voltage Vref, the signal has positive half-periods lasting for 59.3 microseconds, and negative half-periods lasting for 4.7 microseconds. The ratio between these periods is then 12.6. It is noted that, when the black level B varies about the reference value Vref, the time ratio abruptly increases from 4.3 to 12.6.
By selecting an arbitrary value, ranging from 4.3 to 12.6, for the ratio between the charging and discharging current of capacitor C (in the present example, 7), the black level B is always ultimately adjusted with voltage Vref.
FIG. 3 illustrates in more detail the operation of the device of FIG. 2. For the sake of simplicity, FIG. 3 represents a signal CVBSi to be adjusted having an active portion at a constant level. The corresponding adjusted signal CVBSo is represented at a steady state. Each positive half-period of signal CVBSo has, as compared with the corresponding portion of signal CVBSi, a negative slope. This slope corresponds to the discharging of capacitor C with a current I (switch K is open). In contrast, each negative half-period of signal CVBSo has a positive slope 7 times as high as the slope of the positive half-periods. This slope corresponds to the charging of capacitor C with a current 7I (switch K is closed).
Each negative half-period ends before the beginning of the next active portion, but lasts longer than a synchronization pulse S. During the transition interval, signal CVBSo oscillates about value Vref. Accordingly, if the considered slopes are low, the black level B of signal CVBSo is adjusted on value Vref.
If the ratio between the charging and discharging current is chosen lower than value 4.3, in the example of FIG. 3, the high value of the active portion of signal CVBSo would be adjusted on value Vref. In the extreme opposite case, if the ratio between the currents is higher than 12.6, the low level of the synchronization pulses would be adjusted on value Vref.
As represented in FIG. 3 in an exaggerated manner, the adjusted signal CVBSo has slanted portions at places where these portions should be horizontal. In practice, the values of capacitor C and of current I are chosen so that the maximum error is approximately 10 mV for a 700-mV maximum amplitude of the active portion of the signal. This choice results from a tradeoff between the adjustment speed of the signal and a tolerable error.
FIG. 4 represents a portion of the video signal CVBSi to be adjusted, corresponding to a frame retrace Fr. The corresponding adjusted signal CVBSo is also shown. In the vicinity of a frame retrace, a video signal has an active portion with a zero amplitude. The frame retrace includes pulses that successively occur at twice the frequency of pulses S. In the central portion of the frame retrace, corresponding to a frame synchronization pulse train Sf, pulses widen out so that the signal duty cycle is close to 0, whereas the signal duty cycle is close to 1 elsewhere.
As represented by the waveform of the adjusted signal CVBSo, doubling the frequency at the beginning of the frame retrace does not impair the adjustment of the black level (the ratio between the charging and discharging currents is adapted to the duty cycle, close to 1, of the signal). In contrast, during the pulse train Sf, the ratio between the charging and discharging currents is no longer adapted to the duty cycle, close to 0, of the signal. Signal CVBSo progressively shifts upward until its minimum value is adjusted with voltage Vref. At the end of the pulse train Sf; the ratio between the charging and discharging currents is again adapted to the duty cycle of the signal that progressively shifts downward to recover its initial state. However, the compensation time duration for signal CVBSo to recover its initial state is particularly slow and lasts during several lines after the frame retrace. The synchronization pulses of the first lines of the frame are shifted too high to be detected.
A drawback of this shift is that the start-up of the PLL that is adjusted by the synchronization pulses is delayed. This drawback is particularly detrimental when the video signal is provided by a magnetic tape recorder because the synchronization pulses do not successively occur strictly periodically and because the PLL must then react as fast as possible so as to be readjusted with these synchronization pulses, which is incompatible with a delayed start-up of the PLL. As a result, there is a visible distortion at the beginning of the television picture. In the case of a standardized video signal, this drawback is not a major impairment because the synchronization pulses are in phase with the quiescent frequency of the PLL.
Another drawback of such a signal shift is that the information of the first lines cannot be used. Generally, the first lines do not convey picture information, but they can convey teletext information that may be corrupted.